The present invention relates to a procedure known as laser-assisted myocardial revascularization, and more particularly, to improved methods for revascularization of the heart by creating a plurality of small pathways or channels through predetermined portions of the heart using laser energy delivered via a laser delivery means according to specific parameters, including variable frequency, and without requiring synchronization of laser energy delivery with the beating of the heart.
Much of the heart consists of a special type of muscle called myocardium. The myocardium requires a constant supply of oxygen and nutrients to allow it to contract and pump blood throughout the vasculature. One method of improving reduced myocardial blood supply is called trans myocardial revascularization (TMR), the creation of pathways or channels into the myocardium generally from either an outer epicardial surface of the heart in a surgical procedure or from an inner endothelium cell covered surface of a heart""s endocardium chamber in a percutaneous transluminal myocardial revascularization (PTMR).
A procedure using needles in a form of myocardial acupuncture was used clinically in the 1960s. Deckelbaum. L.I., Cardiovascular Applications of Laser Technology, Lasers in Surgery and Medicine 15:315-341 (1994). The technique was said to relieve ischemia by allowing blood to pass from the ventricle through the channels either directly into other vessels perforated by the channels or into myocardial sinusoids which connect to the myocardial microcirculation. These sinusoidal communications vary in size and structure, but represent a network of direct arterial-luminal, arterial-arterial, arterial-venous, and venous-luminal connections. Interest in miyocardial acupuncture or boring, which mechanically displaces or removes tissue, decreased when it was discovered that the mechanically created channels closed because of acute thrombosis followed by organization and fibrosis of clots.
By contrast, recent histological evidence of patent, endothelium-lined tracts within pathways created with laser energy supports the assumption that the lumen of the laser pathways is or can become hemocompatible and resist occlusion caused by thrombo-activation and/or fibrosis. A thin zone of charring occurs on the periphery of the laser-created transmyocardial channels through the well-known thermal effects of optical radiation on cardiovascular tissue. This type of interface may inhibit the immediate activation of the intrinsic clotting, mechanisms because of the inherent hemocompatibility of carbon. In addition, the precise cutting, action that results from the high absorption and low scattering of laser energy (CO2, Ho, etc.) may minimize structural damage to collateral tissue, thus limiting the tissue thromboplastin-mediated activation of extrinsic coagulation. Recent histological studies show that both patent and non-patent channels promote growth of an alternate circulation, one of the mechanisms believed to be beneficial following the procedure. Despite the creation of patent channels and pathways with lasers, there are reported problems associated with laser TMR procedures. Such problems can include channel closure which may be caused by selection and use of TMR laser parameters which do not produce channels with the characteristics detected in the histological evidence discussed above. An additional reported problem encountered in TMR procedures is adverse effects created by the laser on the diseased hearts of TMR patient""s.
U.S. Pat. No. 4,658,817 issued Apr. 21, 1987 to Hardy teaches a method and apparatus for TMR using a surgical CO2 laser including a handpiece for directing a laser beam to a desired location. Hardy suggests that the creation of TMR channels using a laser may affect contractility of the heart and states that the number of perforations may have to be limited accordingly.
Two subsequent patents, U.S. Pat. No. 5,380,316 issued Jan. 10, 1995 and U.S. Pat. No. 5,389,096 issued Feb. 14, 1995 both to Aita et al., discuss in general methods for intra-operative and percutaneous myocardial revascularization, respectively. Both patents suggest synchronization of the laser with the heart beat is necessary to avoid arrhythmias. PCT WO 96/35469 issued Nov. 14, 1996 to Aita et al. also discusses apparatus and general methods for percutaneous myocardial revascularization synchronized with the heart beat to avoid arrhythmias.
Synchronization of the laser energy delivery with the beating of the heart was also considered an important tool in U.S. Pat. No. 5,125,926 issued Jun. 30, 1992 to Rudko et al., reportedly to reduce the chance of laser induced fibrillation. Rudko et al teaches a heart-synchronized pulsed laser system for TMR. Utilizing electrical sensing, the heart beat is monitored using an EKG device. The device automatically delivers what appears to be a square pulse of laser energy to the heart only in response to electrical detection of a predetermined portion of the heartbeat cycle.
The prior art discussed above suggests that at least some pulsed laser systems and parameters are potentially damaging to the beating heart or its action and may induce fibrillation or arrhythmia, hence the need for heart synchronization to minimize such effects.
An arrhythmia is a disturbed heart rhythm which often takes over as the primary rhythm of the heart, as evidenced by a rapid flutter or other rhythm of the heart muscle, which renders it ineffective at pumping blood through the vasculature. The process of delivering laser energy to tissue results in polarization of individual cells of the heart in the area of delivery of the laser energy. Polarization of the specialized conducting cells as well as myocardial cells drives the action potential of cells resulting in responsive contractile motion. Delivering laser energy can disrupt the normal rhythm of the heartbeat since the cardiac rhythm can be side-tracked to that of the polarized cells as opposed to propagating through the heart along the normal path of the impulse.
The heart""s natural, primary pacemaker is found in a group of cells called the sinoatrial or sinus node located near the junction of the superior vena cava and the right atrium. The electrical impulse originates in the endocardium and propagates through the myocardium to the epicardial surface. The electrical impulse is conducted out of the sinus node to the atria, where it stimulates atrial muscle cells to contract, and to the atrioventricular node. Upon leaving the atrioventricular node, the electrical impulse continues to propagate down the conducting system to the bundle of His, into right and left branches thereof. The right bundle spreads the electrical impulse to the right ventricle and the left bundle branch propagates the impulse to anterior and posterior positions in the left ventricle to reach the Purkinje fibers. These small fibers form a rapid conduction network through the myocardium to deliver the impulse to all of the individual contractile muscle cells of the myocardium.
The electrical signal travels at different speeds at different parts of the network. While electrical signals on the portion of the network extending through the atria have been found to travel at velocities of about 1 meter per second, these signals slow to about 0.2 m/s as they pass through the atrioventricular node. Signal propagation through the ventricular Purkinje network, however, is much fasterxe2x80x94approximately 4 m/s. Thus, the sinus node is responsible for producing a repeating electrical impulse which ultimately causes the muscle cells of the heart to contract in repetitive, wave-like convulsions.
The synchronization solutions proposed in the prior art discussed above do not address methods for detecting and compensating for hard to detect, abnormal conduction patterns or rhythms which may occur in damaged hearts. Additionally, EKG monitoring may not detect and allow compensation for localized or isolated areas of heart tissue which may not be synchronized with other areas of heart tissue. Excitation of such isolated areas may cause arrhythmias. In addition to the problems discussed above, heart synchronization as described in the prior art limits the amount of time the laser can be activated during a heart cycle, thereby increasing the time of a TMR procedure.
A need exists in the prior art for a method and apparatus for performing TMR and PTMR procedures quickly using specified laser parameters selected to minimize possible cardiac arthythmias without the need for monitoring the heart beat.
Thus, it is an advantage of the present invention to provide a method for performing both transmyocardial revascularization (TMR) and percutaneous transluminal myocardial revascularization (PTMR) with laser energy having parameters selected to avoid cardiac arrhythmia.
A method for TMR and PTMR with laser energy having parameters selected to avoid cardiac arrhythmia comprises the following steps, in combination of: determining a wavelength of the laser energy from a laser selected to perform either TMR or PTMR; using the wavelength determination to select parameters for the laser energy to produce a non-square wave shape; generating the laser energy at the determined wavelength with the selected parameters to produce the non-square wave shape; and delivering the generated laser energy in one or more pulses to selected portions of heart tissue to perform either TMR or PTMR in the myocardium without inducing cardiac arrhythmia and without synchronizing delivery of the laser energy to a cardiac cycle site. When using a Holmium:YAG laser in either a TMR or PTMR procedure, the selected parameters are power level, energy flux, pulse width, and pulse frequency. In a TMR or PTMR procedure, the laser energy has a wavelength of between about 1.8 and about 2.2 microns, an energy flux of between 0.7-1.78 J/mm2 and a power level of at least about 3 watts, the laser energy being delivered with a pulse frequency of at least about 5 Hertz and a pulse width of between about 150 and about 350 microseconds, the laser energy as delivered causing about 0.5 millimeters or less lateral necrosis surrounding a TMR treatment. When using a Xe:Cl excimer laser in either a TMR or PTMR procedure, the laser energy has a wavelength of about 0.308 microns, a power level of between 0.3-2.0 watts and an energy flux of between about 25-80 mJ/mm2, and is delivered with a pulse frequency of between about 5-25 Hz and a pulse width of between about 20-200 nanoseconds, and causes about 5 microns lateral necrosis surrounding the TMR channel produced thereby. When using a CO2laser for the TMR procedure, the laser energy has a wavelength of about 10.6 microns, an energy flux of about 51 J/mm2 and a power level at least about 800 W, is delivered in a single pulse about of 0.05 seconds and can be gated, and causes between about 0.03 to about 0.2 millimeters lateral necrosis surrounding a TMR channel produced. When using an Argon laser for TMR, the laser energy has a wavelength of between about 0.488 and about 0.514 microns, an energy flux of about 1.3-12.74 J/mm2 and a power level at least about 1-10 W, is delivered in a single pulse, and causes approximately 4 mm lateral necrosis surrounding a TMR channel produced thereby, and is generated by an Argon laser. When using a Nd:YAG laser in a TMR procedure, the laser energy has a wavelength of about 1.06 microns, an energy flux of about 9.5-13 J/cm2 and a power level at least about 2-100 W, is delivered with a pulse frequency of about 1-10 Hz and a pulse width of about 10 nanoseconds, and causes at least about 15 mm lateral necrosis surrounding a TMR channel produced thereby. When using an Er:YAG laser for the TMR procedure, the laser energy has a wavelength of about 2.94 microns, an energy flux of about 50-500 J/mm2, is delivered with a pulse frequency of about 1-15 Hertz and a pulse width of about 1-250 microseconds, and causes about 0.1 millimeters lateral necrosis surrounding a TMR channel produced by an Er:YAG laser.
In a preferred embodiment, the laser energy is delivered to the selected portions of heart tissue using a catheter apparatus with laser delivery means, the method further comprising the following steps of introducing the catheter apparatus with laser delivery means percutaneously into the vasculature of the patient; and positioning the laser delivery means at the endocardial surface of the selected portions of heart tissue. In a preferred embodiment, the laser energy is delivered to the selected portions of heart tissue in a procedure using laser delivery means where the revasculature site is accessed by positioning the laser delivery means at an endocardial surface of the heart tissue through inside a patient""s coronary artery. In a preferred embodiment, the method further includes the step of mechanically piercing the endocardial and/or myocardial layer heart tissue prior to delivering the laser energy into the myocardium thereby creating a welling affect by the surrounding endocardial tissue. In a preferred embodiment, the method further includes mechanically piercing the endocardial surface adjacent the selected portions of heart tissue prior to delivering the laser energy into the myocardium and penetrating no more than half the wall thickness of the myocardium.
The TMR method comprises the following steps of generating laser energy having a non-square wave shape, a selected wavelength, a selected energy flux and a selected power level; and delivering the laser energy in a plurality of pulses, the plurality of pulses having a selected pulse frequency and a selected pulse width, to selected portions of myocardium without cardiac arrhythmia and without synchronizing delivery of the laser beam with the cardiac cycle. In a preferred embodiment, a variable number of pulses of laser energy is delivered with a variable pulse frequency between 5-20 Hz. In a preferred embodiment, the laser energy is delivered with a variable pulse repetition rate of between about 1 and 10 pulses. In a preferred embodiment, the laser energy is delivered with a constant pulse frequency of between about 5 and 20 Hertz and a variable pulse preset limit of between about 1-10 pulses. In a preferred embodiment, the laser energy is delivered in a pulsed mode at a high repetition rate of fixed frequency, the method using a laser with an optical shutter and in which the shutter of the laser is opened and closed in response to a random sequence of commands. In a preferred embodiment, the pulsed laser energy is delivered in a pulsed mode pulsed at a high repetition rate of fixed frequency, the method using a laser with a controllable flashlamp and in which the flashlamp is allowed to fire only during certain pulses within the fixed frequency laser operation in response to a random sequence of commands. In a preferred embodiment, the laser energy is delivered in a pulsed mode pulsed at a random, variable frequency rate.
It is a further advantage of the present invention to provide a method of selecting laser parameters for performing laser-assisted TMR or PTMR procedure to avoid cardiac arrhythmia and without synchronization of delivery of laser energy to a patients s cardiac cycle. The method comprises the following steps, in combination: selecting a minimum power level of laser energy to be used, the minimum power level being sufficient to ablate heart tissue; setting a pulse frequency as great as possible and selected to avoid summation effects; setting a pulse width as long as possible and selected to prevent excessively high peak power without causing undesired levels of thermal damage during TMR or PTMR; shaping a front end of each pulse of laser energy to provide non-linear pulses to avoid cardiac arrhythmia during TMR; and correcting the selected power level, pulse width, pulse frequency, pulse width, and shaping for mechanical events. In a preferred embodiment, the selected parameters are a single pulse, power level, energy flux, and pulse width. Numerous other advantages and features of the present invention will become readily apparent from the following detailed description of the invention and the embodiments thereof, from the claims and from the accompanying drawings in which the details of the invention are fully and completely disclosed as a part of this specification.